1). Field of the Invention
This invention relates to an optical filter, and in particular to a temperature compensated wavelength division multiplexing (WDM) optical filter.
2). Discussion of Related Art
Light filters are used for demultiplexing or multiplexing light in optical a fiber networks. A beam of light is transmitted from an input optical fiber through a light filter layer. The light filter layer is made of a material which transmits a selected range of wavelengths of the light and reflects another range of the wavelengths. It may be important to maintain a range of optical frequencies of transmitted light relatively stable for purposes of further processing of the light. There should be relatively little or no shift in selected transmitted wavelengths, in particular with an increase in temperature.
Accordingly, the present invention relates to an optical filter assembly comprising:
a first optical filter having a first and a second face, one of which is exposed to an incident beam of light, the first optical filter having a refractive index and being of a filter material that transmits a selected transmission range of wavelengths defining a center wavelength, and that reflects a reflection range of the wavelengths, a change in ambient temperature of the first optical filter causing a change in the refractive index with a corresponding change in the center wavelength of the transmission range; and
a first counteracting ring having a higher coefficient of thermal expansion than the filter material attached to the first or the second face of the first optical filter;
whereby a rise in the ambient temperature of the optical filter assembly results in a given length of the first counteracting ring to expand more than a same length of the first optical filter, which causes stretching of the first optical filter resulting in a reduction in thickness therein with a corresponding decrease in the center wavelength of the transmission range of wavelengths, thereby at least partially compensating for an increase in center wavelength caused by the rise in the ambient temperature of the optical filter assembly.